1. Field of the Invention
The present invention relates to a communication system for connecting at least one telecommunication terminal apparatus and at least one computer device to a switching device.
2. Description of the Related Art
Analog telephone equipment is increasingly being replaced by digital equipment, based predominantly on the ISDN (Integrated Services Digital Network) standard. ISDN is defined by several international digital communication standards that are recognized worldwide by telephone companies. ISDN technology is used to send both speech and data that can include graphics, sound, films and digital signals via public telephone networks. The ISDN standard comprises digital standard transmission protocols, terminals and connection cables. The user is provided with two types of ISDN terminals. The international base terminal (S0) comprises two B channels, each having 64 Kbit/s, and a D channel having 16 Kbit/s. The B channels transmit the useful information. The D channel is used for the signaling. Up to eight telephones or other terminal apparatuses can then be operated at an S0 interface.
Besides the base terminal (S0), the primary multiplex terminal (SZM), which is likewise internationally standardized, is also provided; it comprises 30 B channels and 1 D channel having 64 Kbit/s.
ISDN telephones can be operated directly at public networks or at private branch exchanges (PABX, Private Automatic Branch Exchange). In Germany, public communication networks standardly provide two-lead Uk0 interfaces. In what is known as a network termination (NT), these Uk0 interfaces are converted into a four-lead S0 interface. For normal operation, the network termination requires energy from the public power network. In case of a power failure, the network termination (NT) supplies an emergency-supply-authorized terminal apparatus with energy from the public telephone network. Emergency operation is indicated by a reversal of the supply voltage at the S0 interface.
Terminal apparatuses are preferably connected to private branch exchanges via the Up0 interface. The Up0 interface likewise transmits two B channels and one D channel. The Up0 interface, however, is not internationally standardized. For this reason, in addition to the Up0 interface, there also exist many other manufacturer-specific U interfaces.
Since the ISDN standard is a digital standard, it is particularly easy for computers to be connected to ISDN interfaces via plug-in cards. In contrast to speech transmission via telephone or image transmission using fax machines, computers offer the possibility of transmission of very different data formats. Thus, in the domain of the Internet, a multiplicity of image formats, speech compression methods, and formats for transmitting moving images, are used. The use of computers to send and receive faxes as well known as the prior art. With the corresponding software, a computer connected to a printer and to a scanner replaces a fax machine.
One problem in the contemporary PC world is the lack of flexibility of the interfaces used. A PC is standardly equipped with a keyboard interface, one parallel interface and two serial interfaces (RS-232). The keyboard interface is occupied by the keyboard. One serial interface is allocated to the mouse, and the parallel interface is reserved for the printer. Only the second serial interface is available for additional peripheral devices. In the PC domain, data transmission via a serial interface is limited to a maximum of 115.2 Kbit/s. For this reason, peripheral devices that produce large quantities of data, such as scanners, are connected directly to computer-internal busses, PCI bus or the ISA bus, via additional plug-in cards. However, to use such peripheral devices it is necessary to open the computer and install additional plug-in cards. Another disadvantage of the many different interfaces in the PC domain is the use of many different plug connections. In order to solve this problem, various bus systems are known in the prior art. In contrast to the PCI (Peripheral Component Interconnect) and ISA (Industry Standard Architecture) busses, an SCSI (Small Computer Systems Interface) interface can also be led out from the computer housing, and can in this way be used for the connection of up to seven peripheral devices having high data transmission rates, such as hard disks or scanners. A large number of low-price busses are available, such as the Apple Desktop Bus (ADB), the RS-485 interface, which represents an extension of the RS-232 interface, the Access.bus (A.b), the Connection Highway Interface (CHI), the GeoPort, and, recently, the Universal Serial Bus (USB).
An essential goal in the definition of the USB standard was to provide a low-cost bus system for the connection of external peripheral devices to PCs. The USB bus offers low to medium data transmission rates (up to 12 MBit/s). It is thus very well suited for the connection of a large number of peripheral devices, such as scanners, Personal Digital Assistants (PDAs), keyboards and mice. Up to 127 devices can be connected to the USB bus. In addition, the PCI bus supports plug-and-play functionality. The connecting cables are shielded four-lead lines. Two leads are used for the transmission of a supply voltage of 5 volts. The two other leads are twisted, and are used for signal transmission. For data transmission rates of 1.5 MBit/s, unshielded untwisted cables are sufficient. The plugs are designed in such a way that one terminal apparatus can feed a maximum of 5 amperes into the supply line of the USB bus. The energy supply via the USB bus offers the possibility of producing peripheral devices without power supply units, thus saving costs.
PCs and other terminal apparatuses, such as telephones, can be connected jointly to public telephone networks or also to private branch exchanges. As long as the public telephone network or the private branch exchange provides an interface—such as the S0 interface or the Up0 interface—that permits the connection of several terminal apparatuses, the PC and the terminal apparatus can be operated at the same interface, as indicated in FIG. 3. For reasons of cost, telephones are standardly equipped only with the most necessary functions. The telephone or terminal apparatus in FIG. 3 can thus only send data to, and receive data from, the private branch exchange (PABX). Consequently, in FIG. 3 a communication between PC and terminal apparatus is possible only indirectly, via the private branch exchange (PABX). Additional manufacturer-specific solutions according to FIG. 4 have also been created, in which, for example, the PC is connected with the private branch exchange (PABX) via an RS-232 interface, via a terminal apparatus (TE). The advantage of this solution is that at the PC side it is possible to use an already-existing interface, such as the RS-232 interface. It is disadvantageous, however, that this interface does not have the bandwidth required for complete controlling by the PC.
FIG. 3 shows, in addition, the internal construction of a telephone. A telephone essentially has three user interfaces, namely a microphone (acoustic source), a loudspeaker (acoustic sink) and a keyboard for the dialing process (D channel). These three user interfaces, possibly supplemented by additional input and output units, are connected to the private branch exchange (PABX) or to the public telephone network via the telephone-internal IOM-2 bus (Input-Output Multiplexer) with the Up0/E or S0 interface. The IOM-2 interface has a frame structure for three IOM channels. Each of these IOM channels provides four sub-channels, each having 64 Kbit/s. In the IOM-2 frame structure, 2 B channels (64 Kbit/s), one D channel (16 Kbit/s), one D* channel (16 Kbit/s), one CTRL channel (16 Kbit/s) and 2 IC channels (64 Kbit/s) among other things are applied. The B channels are used for data exchange with the switching center, preferably of speech data. The D channel is used for the exchange of control information with the switching center. The two IC channels are used for the exchange of data, preferably speech data, with additional terminal apparatuses, for example slave phones, and the D* channel and CTRL channel are used for the exchange of control information with additional terminal apparatuses. In the connection with an additional telephone (slave phones), the telephone that is connected with the switching center must be configured as a master phone.
U.S. Pat. No. 4,748,656 discloses an interface arrangement that connects a communication system to a telecommunications terminal device. This interface is implemented by a plugin card in a personal computer that, on the one hand, controls the operation of the connected telecommunications terminal device and, on the other hand, offers services of the communication system. The complete signalling from the communication system is interpreted by the personal computer, converted into suitable control signals and forwarded to the telecommunications terminal device. The data received from the telecommunications terminal device are interpreted and modified in the personal computer. Suitable control and signalling messages are derived therefrom from this data that are then forwarded from the personal computer to the communication system. The connection of further peripheral devices to the interface between the personal computer and the telecommunications terminal device, however, is no more possible than is stand-alone operation of the telecommunications terminal device when the personal computer is turned off.
“Isar—1äβt Daten statt Wasser flieβen”, ELEKTRONIK, Vol. 45, No. 20, 1 Oct. 1996, pages 56–60 describes a semiconductor module both for digital as well as for analog data transmission. This module serves the purpose producing cost-beneficial, passive ISDN PC cards that, parallel to the data transfer with ISDN subscribers, can also communicate with subscribers in the analog network and transmit data. The functions of passive ISDN PC cards that are based on the semiconductor module described in this reference are comparable to those of active cards. In one applied example, the semiconductor module is connected to an ISDN transceiver via an IOM-2 bus and is connected to a PC bus interface via a local bus.